Repository Roadmap for Logical Model Integration

The Proposed
Support for Logical Model Integration document outlines all the areas that
are targeted to be addressed in 3.2 related to logical model integration support
in Eclipse. The purpose of this document is to describe the APIs repository providers
can use to support logical models.

To provide full support for logical models based on the Eclipse 3.2 API,
a repository provider can perform the following steps:

Contribute the appropriate repository operations to elements
that adapt to ResourceMapping.

Ensure that operations performed on resource mappings include
all the appropriate model elements and resources by using an
ISynchronizationScope
and supporting API.

Allow model providers to participate in headless merging through the
IMergeContext
interface and supporting API.

Allow model providers to participate in merge previews by using the
teamContentProviders for the models involved in the merge.
A ModelSynchronizeParticipant class is provided to help manage
the relationship between the model content, a merge context and the Compare
framework.

The following sections describe each of these points in more detail.
A pointer to a simple Library example is also provided.

Contributing Actions to Resource Mappings

The Basic Resource Mapping API

The resource mapping API consists of the following classes:

ResourceMapping: The Class to which logical model elements
adapt to indicate that the model corresponds to a set of resources in the
workspace. The complete ResourceMapping class can be viewed here.
The methods of interest are:

Object getModelObject(): The model object from which the
mapping was derived (or adapted).

ResourceTraversal[] getTraversals(ResourceMappingContext, IProgressMonitor):
The resource traversal that cover the resources that constitute the model
object.

ResourceTraversal: A ResourceTraversal
contains a set of resources and a depth flag that indicates the depth to which
the resources in the traversal are associated with the originating model object.
Resource traversals are provided to a client by a resource mapping in order
to describe the contents of a model in such a way that the client (e.g. a
repository provider) can perform its operations in as efficient a means as
possible. Methods of interest are:

getResources()

getDepth()

ResourceMappingContext: a context that is provided to the
resource mapping by the client when obtaining traversals. This context allows
the logical model to determine what the remote state of the model is so that
the proper resources can be covered by the resource traversals returned by
the resource mapping. The use of theResourceMappingContext
andRemoteResourceMappingContext
is a bit more complicated and is described later.

There are two types of plugins that should be interested in resource mappings.
Those who provide a model that consists of, or is persisted in, resources in
the workspace and those that want to perform operations on resources.
The former case will be covered in a document targetted at model providers and
the later case is covered in the next section.

Resource Mappings and Object Contributions

Plug-ins that contribute extensions to adaptable extension points will have
to make two changes to support the new ResourceMapping APIs:

Update any objectContributions in their plugin.xml file to target
ResourceMapping instead of IResource (for those for which this is
appropriate).

Update their actions to work on ResourceMapping instead of
IResource and respect the depth constraints provided in the traversals.

Plug-ins that add object contributions to IResource
can now add them to ResourceMapping instead,
if the action can apply to multiple resources. Here is an XML snippet that contributes
a menu action to objects that adapt to resource mappings:

Contributions to ResourceMapping will automatically apply to objects
that adapt to IResource. This transitive association is handled
by the Workbench. Filtering of the contributions to resource mappings can be done using enablement
expressions. An expression for filtering by project persistent property
has been added to allow repository providers to have their menus appear on projects
that are mapped to their repositories.

Actions that have been contributed to the ResourceMapping class
will be given a selection that contains one or more ResourceMappings.
It is the actions responsibility to translate the resource mapping into a set
of resources to be operated on. This can be done by calling getTraversals
to get the traversals of the mapping. Traversals are used to allow the clients
of the traversal to optimize their operations based on the depth of the resources
being traversed. A client may traverse the resource manually or may use the
resource and the depth as input into an operation that the action delegates
to do the work. As an example, if the user performs a CVS update on a java package
and the java package resource mapping maps to a folder of depth one, CVS would
issue an appropriate command ("cvs update -l" for those who are curious) which
would perform a shallow update on the folder the package represents.

Although it is possible to obtain a set of traversals directly from the selected
resource mappings, their are model relationships (or repository reltionships) that may
require the inclusion of additional resources or model elements in an operation. The next
section describes how to ensure that all required resources are included in an operation

Operation Scope

For team operations, the selected mappings need to be translated into the set
of mappings to be operated on. This process involves consulting all model providers
to ensure that they get included in operations on resources that match their
enablement rules. The term we use to describe the complete set of resource mappings
to be operated on is the operation scope. The following API has been
provided for this:

ISynchronizationScope:
Interface that defines the API for accessing the scope of the operation. It
provides access to all the resource mappings being operated on and the traversals
for those mappings as they were calculated during the scope building process.

ModelOperation:
Extendable operation class that generates a scope using a provided scope manager and prompts if additional resources or mappings
have been included in the operation due to model provider relationships.

The initialize(IProgressMonitor) method of the SynchronizationScopeManager class handles the entire process of converting
an input set of resource mappings into the complete set of mappings that need to be operated on as well
as the complete set of traversals that cover these mappings. A repository provider can tailor the process
by:

Providing a RemoteResourceMappingContext for use when obtaining resource traversals
from resource mappings.

Overriding SynchronizationScopeManager to tailor the scope management
process as required.

The next two sections describe these points in more detail.

Remote Resource Mapping Context

In order to guarantee that all necessary resources get included in a team operation,
the model provider may need the ability to glimpse at the state of one or more resources
in the repository. For some models, this may not be required. For instance, a java package
is a container visited to a depth of one, regardless of the remote state of
the model. Given this, a repository provider can easily determine that outgoing
deletions should be included when committing or that incoming additions should
be included when updating. However, the resources that constitute some logical
models may change over time. For instance, the resources that constitute a model
element may depend of the contents of a manifest file (or some other similar
mechanism). In order for the resource mapping to return the proper traversal,
it must access the remote contents of the manifest file (if it differs from
the local contents) in order to see if there are additional resources that need
to be included. These additional resources may not exist in the workspace but
the repository provider would know how to make sure they did when the selected
action was performed.

In order to support these more complex models, a RemoteResourceMappingContext
can be passed to the ResourceMapping#getTraversals method. When
a context is provided, the mapping can use it to ensure that all the necessary
resources are included in the traversal. If a context is not provided, the mapping
can assume that only the local state is of interest.

The remote resource mapping
context provides three basic queries:

What type of comparison is being performed: two-way or three-way

for two-way, does the local contents differ from the remote contents

for three-way, are there local changes and are there remote changes

What are the contents of the remote (and base for three-way) counterparts
of the file

What are the remote members of a folder

The answer to the first question above depends on the type of operation that
is being performed. Typically, updates and merges are three-way while comparisons
and replace operations (at least for CVS) are two-way.

The Eclipse Team API includes a Subscriber class that defines
an API for providing the synchronization state between the local workspace and
a remote server. A SubscriberResourceMappingContext
has been created that uses a Subscriber to access the necessary
remote state. Clients that have a Subscriber do not need to do
any additional work to get a resource mapping context.

Subclassing SynchronizationScopeManager

The SynchronizationScopeManager class can be subclassed to tailor the scope
generation and management process. The two main reasons for subclassing the scope manager are:

The repository provider needs to include additional resources due to some repository
level relationship (e.g. change set). This can be accomplished by overriding the
adjustInputTraversals(ResourceTraversal[]) method.

The synchronization has a longer lifecycle (e.g. Synchronize view vs. dialog) and needs the potential to react to scope
changes. The ISynchronizationScopeParticipant
interface defines the API that model providers can use to participate in the scope management process.
The SubscriberScopeManager
class is a Subscriber based subclass of SynchronizationScopeManager that involves participants in the scope management process.
An example of why this type of process is needed is working sets. If a working set is one of the resource mappings in a scope, the set of traversals
covered by the scope would increase if resources were added to the working set.

Model-based Merging

The main repository operation type that requires model participation is merging. In many
cases, models only need to participate at the file level. For this, the IStorageMerger
API was introduced to allow model providers to contribute mergers that should be used to merge
files of a particular extension or content type. However, in some cases, models may need
additional context to participate properly in a merge. For this purpose, we introduced the
IResourceMappingMerger and IMergeContext APIs.

Merge operations are still triggered by actions associated with a repository
provider. However, once a merge type operation is requested by the user, the
repository provider needs to involve the model providers in the merge process to
ensure that the merge does not corrupt the model in some way.

There are two main pieces of repository provider API related to the model-based merging support.

API to describe the synchronization state of the resources involved in the merge.

API to allow model providers to merge model elements.

The following sections describe these two pieces.

API for Synchronization State Description

An important aspect of model-based merging is the API used to communicate the
synchronization state of the resources involved to the model provider. The following
interfaces are used to describe the synchronization state:

ISynchronzationContext:
provides access to the synchronization state of all resources within the scope
of the operation. The following API pieces are used to describe the synchronization
state of resources.

ITwoWayDiff:
A diff that provides more details about a change between two states of an element (CONTENT, MOVE_TO, MOVE_FROM, COPY_FROM, REPLACE).

IThreeWayDiff:
A diff that consists of either a local or remote two-way diff or both and provides a direction description for the change (INCOMING, OUTGOING, CONFLICTING) based on the presense of either or both the local and remote change.

IResourceDiff:
A two-way diff that applies to resources and provides access to the before state and after state of files through the
IFileRevision
API.

IDiffTree:
A generic path-based data strucuture for accessing a tree of diffs.

IResourceDiffTree:
A resource-based data structure that contains the synchronization state for all resources
in the operation scope that are out-of-sync.

Abstract classes are provided for all these interfaces with the convention that the class names match the
interface names with the "I" prefix removed. The only class that repository providers must override is
the ResourceDiff class so that appropriate before and after file revisions can be provided.

Performing a group of operations while holding an appropriate scheduling rule.

An abstract
MergeContext
class is provided that contains default implementations for much of the
merging behavior and also uses the
IStorageMerger
to perform three-way merges.
A
SubscriberMergeContext
class is also provided which handles the population and maintenance
of the synchronization state description associated with the merge context.

An operation class,
ModelMergeOperation
is provided which uses the IResourceMappingMerger API to perform a model-based merge operation. Subclasses need
to overriding the initializeContext(IProgressMonitor) method to return a merge context.
The operation uses this content to attempt a headless model-based merge. If conflicts exist,
the preview of the merge is left to the subclass. As we'll see in the next section, there is a
ModelParticipantMergeOperation
that provides preview capabilities using a
ModelSynchronizeParticipant.

Model Content in Team Viewers

Support for the display of logical models in a team operation is provided using the Common
Navigator framework which was introduced in Eclipse 3.2. Logical models can associate a
content extension with a model provider using the org.eclipse.team.ui.teamContentProvider
extension point. Team providers access these content providers through the
ITeamContentProviderManager.

There are several places where a team provider may wish to display logical models:

When the scope of an operation has been expanded due to model provider relationships.
The ModelOperation class provides a prompt that uses the registered team content providers
to inform the user of a scope expansion.

When a merge preview is required. The
ModelSynchronizeParticipant
class makes use of
the team content providers and a merge context to display a merge preview. Team providers can extend this class
in several ways to tailor the preview.

The ModelSynchronizeParticipant provides integration into the Synchronize view or any container that can display
iSynchronizePages. The participant makes use of both the pre-existing synchronization participant capabilities and the
Common Navigator capabilities to allow for team providers and models to tailor the toolbar, context menu and other aspects of the
merge preview. The ModelSynchronizeParticipant provides the following:

A synchronize page that defines thet link between a scope manager, merge context and team content providers.

File History

A file history API has been added to allow models to access the history of files. The file history API consists of the following interfaces:

IFileHistoryProvider:
Obtained from a RepositoryProvider, the file history provider allows clients
to obtain the file history of a particular file.

IFileHistory:
A data structure that allows access to the history of a file

IFileRevision:
The description of a single revision or state of a file at a particular point in time in its history.

Along with this API, a generic file History view has been added. This will allow Team providers to display their file/resource
history in a shared view and also allows models to display model element history for elements tat do not map
directly to files. The History view is a page-based view which onbtains a page for the selected element in the
following way:

A history page for workspace resources is obtained by adapting the RepositoryProvider associated
with the project that contains thge resource to an
IHistoryPageSource.

A history page for other objects is obtained by adapting the object itself to an IHistoryPageSource

Project Set Capability

Methods have been added to ProjectSetCapability to support the translation between a reference string
used to identify a mapping between a project and remote content and
URIs that identify a filesytem scheme regsitered with the org.eclipse.core.filesystem.filesystems extension point.
Team providers can optionaly provide support for this in order to allow logical models to performing
remote browsing and project loading.

Decorating Model Elements

Team providers can decorate model elements by converting their lightweight decorators to work for resource mappings
in the same way object contributions are converted to work for resource mappings. However, there is one aspect of
logical model element decoration that is problematic. If a model element does not have a one-to-one mapping to a
resource, the model element may not recieve a label update when the underlying resources change.

To address this issue, a SynchronizationStateTester class was added. This class is used by model
views to determine when the labels of logical model elements need to be updated. This API relies on the Subscriber
interface to determine when the team state of resource has changed. To work properly, repository providers
must override the RepositoryProviderType#getSubscriber() method in order to return a subscriber
that provides clients access to the team state of workspace resources.

EMF Library Example

This example, available here, is built on a model that consists of libraries,
books and writers.